1. Field of the Invention
This invention concerns a flight control system for an aircraft. The flight control system of the present invention is primarily intended for use in an unmanned aircraft, but it is also suitable for use in a manned aircraft.
2. Discussion of Prior Art
Conventionally, flight control in an aircraft has been achieved by way of aerodynamic control surfaces, for example the ailerons, the flaperons, the elevators, the spoilers and the rudder, attached to the aerodynamic lift generating surfaces of the aircraft. As the inherent airframe instability of present day aircraft and air speeds have increased, the actuation forces needed to move these aerodynamic control surfaces have greatly increased, and the dynamic response requirements of the control surfaces have likewise increased.
Most aircraft currently employ hydraulic actuators for operating the aerodynamic control surfaces, such actuators being capable of producing high forces within relatively confined spaces.
However, hydraulic actuation systems suffer from a number of significant drawbacks, in particular their weight and their need for a high level of maintenance in order to minimise wear and tear and ensure reliability. Further, in order to meet safety regulations such actuators often have to have capabilities well in excess of the normal requirements of the aircraft. For example, safety regulations may dictate that an actuator have twice the necessary thrust capability that is actually required in normal use of the aircraft. All of this adds to the design complexities and expense of such actuator systems.
Electrically powered actuation systems have also been considered and offer certain advantages in terms of weight, life cycle, reliability and maintenance. However, the electrical actuation systems that are currently available are unable to meet the high power transmission requirements and other demands of present day aircraft primary flight control systems.
It is an aim of the present invention to provide a flight control system for an aircraft, which reduces the control forces and power demands needed for aircraft flight control.
Another aim of the present invention is to provide a flight control system for an aircraft, which avoids the need for complex, high power actuator transmission systems.
The present invention provides an aircraft having a flight control system comprising aerodynamic lift generating means, and means for controlling in use the lift provided by the lift generating means, the lift control means comprising:
a concentrated mass mounted within a longitudinal body of the aircraft, the concentrated mass comprising at least one existing aircraft function and/or system mounted on a support in a confined area,
bearings, permitting relative movement between the support for the concentrated mass and at least a portion of the lift generating means, and
actuation means for causing such relative movement whereby in use the centre of aerodynamic lift and the centre of gravity of the aircraft may be moved relative to one another for effecting flight control.
In a preferred embodiment of the invention described below, the concentrated mass is movable relative to the centre of lift of the aircraft. It is also possible for the concentrated mass to be movable relative to the thrust line of the or a respective aircraft engine.
In a preferred form of the invention described below, the flight control system has at least one concentrated mass mounted within the aircraft body, and the or each mass is movable in two perpendicular directions respectively relative to the aircraft body. For example, the or each concentrated mass may be movable respectively laterally and longitudinally of the aircraft body.
An advantage of this embodiment of the present invention is that the concentrated mass is movable, and entirely located, within the aircraft body in use and therefore there need be no external moving control surfaces, which has stealth benefits.
Further, by the use of low friction bearings, relatively low actuation forces can be employed for moving the concentrated mass, in comparison with the force requirements in a conventional flight control system. As a result, it is envisaged that low power actuation means may be employed, which opens the way to the use of electrical actuators. Existing low power electrical actuators are generally able either to provide a high force or to act at a high rate but not both, and so such actuators have not hitherto been employed in flight control systems. However, the nature and location of the concentrated mass according to the present invention enables linear electrical actuators to be employed with consequent advantages of ease of installation and maintenance, reliability, efficiency and economy.
In an alternative embodiment, the or a portion of the aerodynamic lift generating means is movable relative to the body of the aircraft. For example, a wing or wings of the aircraft may be shiftable relative to the body.
Such an arrangement may have the advantage of simplicity over an arrangement involving movement of a concentrated mass within the interior of the aircraft frame given the complex structuring of such interior space with its need to accommodate various aircraft systems, stores, fuel, engine, etc.
It is envisaged that the concentrated mass may comprise at least one of: the aircraft actuation system, the aircraft fuel tanks and the aircraft storage containers close packed together in a confined space.
According to a further aspect of the invention, there is provided a method of flight control in an aircraft having aerodynamic lift generating means by controlling in use the lift provided by the lift generating means, said lift control comprising:
providing a concentrated mass mounted within a longitudinal body of the aircraft, the concentrated mass comprising at least one existing aircraft function and/or system mounted on a support in a confined area, and
effecting relative movement between the concentrated mass and at least a portion of the aerodynamic lift generating means whereby to shift the centre of gravity and the centre of aerodynamic lift of the aircraft relative to one another for generating lift controlling moments about said centre of gravity.
According to another aspect of the present invention, the aircraft may in addition to or instead of the flight control system mentioned above employ wing area control for flight control purposes.
According to this aspect of the invention, an aircraft has a pair of wings, each having a variable wing area.
In a preferred form of this aspect of the invention described below, each wing comprises a first wing portion which is fixed relative to the body of the aircraft, a second wing portion which is mounted to the first wing portion by means of bearings so as to be extendable and retractable thereto, and actuation means for extending and retracting the second wing portion.
Preferably, the second wing portion is telescopically received within the first wing portion.
An advantage of this aspect of the invention is that the wing area can be adjusted before and/or during flight to control the lift of the aircraft and to suit particular flight conditions.
For example, a larger wing area can be employed for take-off and for high altitude loiter, whereas a smaller wing area can be employed for low altitude high speed flight or for flying in turbulent weather conditions in order to enhance flight control.
According to a further aspect of the invention, there is provided a method of flight control in an aircraft having wings comprising: varying the wing area for at least one of the wings for controlling the lift provided by the wing.